The biological contact stabilization process has been applied in sewage treatment for many years. However, the conventional terrestrial biological contact stabilization process is disadvantageous in that it fails to endure low temperature, tends to cause drying and accordingly death of biological films when the supply of water stops, and tends to produce bad odor and result in the propagation of flies and mosquitoes when the aeration is insufficient. Therefore, the submerged biological contact stabilization process has been developed and widely applied in sewage and wastewater treatment.
On the other hand, the conventional biological aerated lagoon has also been applied in the sewage treatment for more than one hundred years. In the past years, improvements made to the biological aerated lagoon lie mainly in the aerator used therewith and the mixing of water without other significant breakthrough. Since the biological aerated lagoon does not provide the function of sludge recycling, the mix liquor suspended solid (MLSS) concentration in the biological aerated lagoon is at best about 500 mg/l. Thus, the conventional biological aerated lagoon requires a very large area because a holding time as long as 10 to 30 days is required. Further, the MLSS in the biological aerated lagoon is subject to poor settling due to changes in water temperature in different seasons, which would result in excessive suspended solids in the discharged water and accordingly, poor sewage treatment effect.
Therefore, it is desirable to develop an improved submerged biofiltration purifying apparatus, so that sewage treatment can be performed in a simple way at reduced electric power consumption to provide stable filtering effect, upgraded pollutant removal efficiency, and high flexibility in operation.